In one known type of optical sensor, an encoder is attached to a movable member, and the position of the member is determined by optically interrogating the encoder. The member may be one that rotates, in which case the encoder may comprise a disk that rotates with the member, or may be linearly movable, in which case the encoder moves linearly along with the member. In an analog encoder, the encoder includes a track that has a continuously variable optical property, such as a continuously variable transmission coefficient. In a digital encoder, on the other hand, the encoder includes a number of parallel coded tracks, each of which represents a specific bit of a binary word. For each position of the encoder, the tracks will present a different binary word to the optical interrogation system.
In the past, analog encoder tracks have typically been manufactured by depositing a variable density metallic attenuating film on a transparent disk or plate. The linearity of commercially available analog encoders manufactured by such a technique has generally been limited to about 5%. Analog optical encoders have therefore not been competitive with digital encoders, where the precision is limited only by the highest achievable bit density of the least significant track.
In a fiber-optic analog encoder system, optical energy is transmitted to and from the encoder by optical fibers. Optical fiber couplers are used to connect the optical fibers to one another, and to the other components of the sensor system. The couplers allow each fiber path to be constructed from several discrete lengths of optical fiber. Such couplers have insertion losses that are neither negligible nor exactly reproducible. Since the optical attenuation of the fiber-optic link is an unknown and variable factor, the attenuation of the fiber link must be known before the position of the encoder can be measured solely friom an end-to-end attenuation measurement. Alternatively, the optical modulation technique inside the sensor must be such that the position of the encoder can be found independently of the fiber link losses.
One method of making an attenuation measurement inside the sensor that is independent of fiber link losses employs wavelength division multiplexing. Light of two or more wavelengths is launched into the outgoing fiber, and is demultiplexed inside the sensor. After demultiplexing, light at each wavelength is differentially modulated, recombined, and then transmitted back to a receiver along a single return fiber. At the receiver, light of the two different wavelengths is demultiplexed to yield the intensities at the two or more wavelengths. After suitable signal processing, the attenuation due to the encoder track, and hence the position of the movable member, may be found. When light of two separate wavelengths is used, the system is known as a two-wavelength, referenced sensing system.